1. Field of the Invention
The present invention relates to a waveguide light detecting element, and particularly to a waveguide light detecting element used in an optical communication system or the like.
2. Description of the Related Art
With a leap increase in amount demanded for communications, an attempt to increase the capacity of a communication system has been made. To this end, however, there has been a need to speed up an optical communication apparatus and bring it into less size/high efficiency and less cost.
In an optical communication transmission system, lights lying in two wavelength bands have been used as signal lights. One of them is a signal light lying in a 1.3 μm band in which a center wavelength of a bandwidth of the signal light is 1.3 μm, and the other thereof is a signal light lying in a 1.55 μm band in which a center wavelength of a bandwidth of the signal light is 1.55 μm.
The 1.55 μm-band signal light is low in optical fiber loss and used as a signal light for a long-distance communication system. This is called interurban communication (trunk-line system) and is used for communications between large cities as in the case of Tokyo-to-Osaka, for example.
On the other hand, the 1.3 μm-band signal light is large in optical fiber loss but less in wavelength dispersion and is used as a signal light for a short-distance communication system. This is called an in-city communication and has been used in large city communication. Also the 1.3 μm-band signal light is used even in communications between an access system a base station and respective homes.
In an optical communication system, as a photodiode for receiving the signal lights lying in the two wavelength bands, a waveguide type semiconductor photodiode adaptable to one wavelength, which has been formed so as to be adapted to the lights lying in the respective wavelength bands, had been used.
As a known example of a conventional waveguide type light receiving element, there has been disclosed a configuration wherein an n-InGaAsP optical guide layer (bandgap wavelength: 1.3 μm), an InGaAs optical absorbing layer, a p-InGaAsP optical guide layer (bandgap wavelength: 1.3 μm), and a p-InP layer are sequentially laminated over an n conductivity type InP substrate (hereinafter an n conductivity type is represented as “n-”, a p conductivity type is represented as “p-”, and an intrinsic semiconductor is represented as “i-”, respectively) (see, for example, section 0001 in Japanese Patent Application Laid-Open No. H10-125948).
As another known example, there has been disclosed a semiconductor light receiving element having a configuration wherein a material small in bandgap adapted to a 1.3 μm band and a 1.5 μm band is used for an optical absorbing layer to receive lights lying in the 1.3 μm band and 1.5 μm band well used in optical fiber communications, and a guide layer of n-InAlGaAs or n-InGaAsP, an avalanche-doubling layer of n-InAlAs, an electric field relaxation layer of p-InAlAs or p-InP, a low-concentration optical absorbing layer of p-InGaAs, a high-concentration optical absorbing layer of p-InGaAs, a p-type guide layer, and a p-type contact layer are sequentially laminated over an n-InP substrate to form a mesa stripe-shaped waveguide, which is covered with a passivation film of SiOx or SiNx (see, for example, sections 0023 to 0025 in Japanese Patent Application Laid-Open No. H11-330536).
As a further known example, there has been disclosed a mesa type having a double-core structure of an InGaAlAs system as a configuration of a 1.5 μm-light receiving wavelength band 10 Gb/s waveguide type PIN-PD, wherein In0.53Ga0.47As is used in an optical absorbing layer (see, for example, “Characteristics of light receiving wavelength-1.5 μm band 10 Gb/s waveguide PIN-PD”, (The 50th Spring Lecture Proceedings (Kanagawa University, 2003.3), 2003 (Heisei 15th Year)); The Japan Society of Applied Physics, p.1246, 27p-H-15).
The conventional waveguide type light receiving element is configured as a photodiode adapted to a signal light lying in a single wavelength band used in its optical communication system. However, there is a possibility that a communication network maintained for in-city communications will be used as for interurban communications at this stage with enlargement of transmission capacity in the optical communication system. In this case, the optical communication apparatus employed in the optical communication system should be unavoidably complicated in configuration where optical parts adapted to respective wavelengths are used as at the present time.
However, even if an optical part adapted to the signal light of one wavelength, here a waveguide type photodiode (hereinafter called waveguide PD), the waveguide PD adapted to the one-wavelength signal light has received two-wavelength signal lights as it is, it was difficult to cause the waveguide PD to perform a high-speed operation at high sensitivity.
That is, the waveguide PD has a structure wherein light is confined in a waveguide portion having an optical absorbing layer and optical guide layers provided with the optical absorbing layer interposed therebetween, and the light is absorbed while the light confined in the waveguide portion is being propagated to the optical guide layers and the optical absorbing layer, and converted into an electric signal.
This waveguide PD confines the light in the waveguide portion and allows the waveguide portion to absorb the light by use of the difference in refractive index between the optical absorbing layer and each of the optical guide layers and cladding layers. Therefore, when the signal lights are different in wavelength from each other, the optical absorbing layer, optical guide layers and cladding layers adapted to the respective lights are different from one another in refractive index.
Thus, the waveguide PD corresponding to the single wavelength band is capable of optimizing a device structure in conformity with a light-receiving wavelength band. However, it happens that the waveguide PD corresponding to a multiwavelength is excellent in sensitivity characteristic at a certain wavelength but very poor in sensitivity characteristic at another wavelength. It can also happen that the sensitivity characteristic is degraded in all the wavelength bands in some cases.
For instance, as it is able to increase or improve confining of light in a waveguide by enlarging the difference in refractive index between an optical guide layer and a cladding layer, a composition wavelength on the long-wavelength side may preferably be selected from composition wavelengths in each of which a bandgap signal light passes through the optical guide layer, as the composition wavelength of the optical guide layer.
However, in order to cope with the multiwavelength, the optical guide layer must have a composition wavelength in which a signal light lying in the shortest wavelength band can pass through its corresponding optical guide layer. It can happen that if the composition wavelength of the optical guide layer is simply determined on the basis of the wavelength of the signal light lying in the shortest wavelength band, sensitivity is significantly degraded with respect to signal lights lying in other wavelength bands.
Thus, a problem arises in that even if the waveguide PD high in sensitivity and capable of high-speed operation corresponding to the signal light lying in the first wavelength band has received signal lights lying in a second wavelength band or other wavelength bands as it is, the waveguide PD encounters difficulties in enabling high sensitivity and high-speed operation with respect to these signal lights.